Nebular metallicities in isolated dwarf irregular galaxies

Description

The motive for this work was to investigate whether small, isolated gas-rich galaxies show
evidence of chemical evolution, by studying their nebular metallicities. I have identified
a sample of 83 objects chosen for low luminosity and mass, the presence of active star
formation, and isolation from other galaxies and galaxy clusters that might generate tidal
effects or enrich the intergalactic medium. From these I have measured the spectra of 35
objects, using theWiFeS IFU spectrograph on...[Show more] the ANU 2.3m telescope at Siding Spring.
In analysing spectra extracted from the WiFeS data cubes, I found that standard ‘strong
line’ methods using emission line ratios to measure atomic abundances, gave either erratic
or no results. I found that for those galaxies showing the [O iii] 4363Å auroral line, the
metallicities determined using the standard ‘electron temperature’ methodwere inconsistent
with previous published work. This led me to investigate the conventional assumption that
electrons in Hii regions are in thermal equilibrium. I show that the non-equilibrium ‘ ’
electron energy distribution, found almost universally in solar system plasmas, can explain
the long recognised ‘abundance discrepancy’ between recombination line and collisional line
abundance calculations in nebular metallicity measurements. This has added an important
new dimension to the analysis of nebular spectra.
Using the extensively revised Mappings photoionisation modelling code and new atomic
data to analyse the spectra of two exceptionally isolated dwarf galaxies, I find that they
exhibit metallicities similar to galaxies in more crowded environments, and appear to have
evolved quite normally, through periodic star formation and subsequent enrichment of
their interstellar media.
I present a new approach for calculating total oxygen abundance using electron temperatures
that appears to give more consistent results than earlier methods. I apply this to my
measured spectra, together with the revised Mappings photoionisation modelling code, to
explore the physical parameters affecting the measurement of nebular metallicities. In particular,
I find strong evidence for several of the observed nebulae being—in part—optically
thin. I use the models to show that nebular optical depth affects measured abundances and
temperatures, and that electron densities also have an important role. I develop models that
give a very good match to the observations.
I conclude that the measurement of abundances and temperatures in Hii regions is a more
complex question than had generally been assumed, and important physical parameters
affecting the measurement processes have in the past not been taken fully into account.